Various dehydrogenation processes have been proposed to dehydrogenate dehydrogenatable hydrocarbons such as cyclohexanone and cyclohexane. For example, these dehydrogenation processes have been used to convert at least a portion of cyclohexanone into phenol.
Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone.
Other known routes for the production of phenol involve the direct oxidation of benzene, the oxidation of toluene, and the oxidation of s-butylbenzene wherein methyl ethyl ketone is co-produced with phenol in lieu of acetone produced in the Hock process.
Additionally, phenol can be produced by the oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide wherein cyclohexanone is co-produced with phenol in lieu of acetone produced in the Hock process. A producer using this process may desire to dehydrogenate at least a portion of the cyclohexanone produced into the additional phenol depending on market conditions.
Thus in International Patent Application WO 2010/024975 filed Jul. 14, 2009, a process has been proposed for producing phenol by oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide followed by cleavage of the cyclohexylbenzene hydroperoxide, in which at least a portion of the effluent from the cleavage step is subjected to a dehydrogenation step. The dehydrogenation not only converts at least a portion of the cyclohexanone in the effluent portion to additional phenol but also generates hydrogen as a by-product, which can, for example, be recycled to an initial benzene hydroalkylation step for producing the cyclohexylbenzene feed. In addition, although the cleavage effluent portion subjected to the dehydrogenation step can be a substantially pure cyclohexanone fraction produced by separation of the phenol and light and heavy ends from the raw effluent, given the cost of this separation, the process can also be applied to an effluent portion containing some or all of the phenol produced in the cleavage step. In this way, the total cost of purifying the final phenol stream and the amount, if any, of the final cyclohexanone stream can be minimized.
Currently, however, the viability of this method of controlling the cyclohexanone content in the product of the Hock process via cyclohexylbenzene is the stability of the dehydrogenation catalyst, since most existing catalysts capable of promoting the dehydrogenation of cyclohexanone to phenol deactivate rapidly and hence require frequent reactivation and/or replacement. Surprisingly, it has now been found that a significant improvement in catalyst stability in the dehydrogenation of cyclohexanone can be achieved by co-feeding at least one of cyclohexane, cyclohexene and benzene with the cyclohexanone. This is an important discovery, particularly in the case of cyclohexane, since cyclohexane is a major and currently unwanted by-product of the initial benzene hydroalkylation process. Under the conditions of the cyclohexanone dehydrogenation step, the cyclohexane is converted via cyclohexene to benzene, which can be recycled to the benzene hydroalkylation step. Thus the present method not only improves the cyclohexanone dehydrogenation step but also addresses a significant problem of the benzene hydroalkylation step.